1. Field of the Invention
The present invention relates to oscillators, and it particularly relates to an oscillator in which the frequency of oscillation can be varied.
2. Description of the Related Art
A voltage controlled oscillator is used in optical pickups or phase-locked loops (PLL), for instance. Generally, an oscillation frequency is adjusted according to a control voltage applied and then a signal of the thus adjusted oscillation frequency is outputted. In an example of a conventional voltage controlled oscillator, an inverting amplifier, a first charge/discharge circuit and a second charge/discharge circuit are electrically coupled to form a circuit. In this structure, the phase of an inverted voltage signal from the inverting amplifier is delayed in stages at the first and the second charge/discharge circuit and the output of the second charge/discharge circuit is again inputted to the inverting amplifier. After a full circle, the phase of an inverted voltage signal returns to the same phase as the original one, so that the voltage controlled oscillator can keep oscillating by repeating the above processing. The oscillation frequency of the voltage controlled oscillator is determined mainly in response to the magnitude of the charge/discharge current at the first and the second charge/discharge circuit, and the magnitude of the charge/discharge current is controlled by an easily controllable control current which has a current value level higher than the charge/discharge current (See, for example, Reference (1) in the following Related Art List).